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Study Validates AACC Recommendation Against Using Ct Values for SARS-CoV-2 PCR Tests

NEW YORK – A new study published on Monday in the American Association of Clinical Chemistry's journal Clinical Chemistry reinforced AACC's recommendations against using cycle threshold values to determine the performance of SARS-CoV-2 PCR tests.

Earlier this year, AACC issued a public statement advising against the use of Ct values for PCR tests when managing COVID-19 patients. It noted that validation of a quantitative test is more complex and requires more rigorous testing to ensure performance, such as determining the lower limit of quantification rather than the lower limit of detection.

The independent study, led by researchers at the UK National Measurement Laboratory at LGC, looked at more than 6,000 patients who were tested for SARS-CoV-2 with PCR at three labs in the UK, Belgium, and South Korea.

The team used the World Health Organization's suggested Ct cutoff of 25 for the test results and found that clinical sensitivity dropped from 100 percent to between 16 percent and 90 percent, depending on the patient cohort.

Looking at an additional 732 labs, the researchers found that an individual Ct value can correspond to different viral loads depending on the lab.

Jim Huggett, the leader of the team, said in a statement that although Ct values may be useful for epidemiological assessments, "they should be avoided as a quantitative measure for individual patient stratification or … analytical performance targets."

"If quantification is to be performed, copy-based units calibrated to appropriate standards should be explored," he said, adding that for a new pathogen, such standards may not be available yet and should be produced quickly as "an important part of [a] diagnostic response plan to a new epidemic."

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